Chemical gold plating composition



United States Patent 3,396,042 CHEMICAL GOLD PLATING COMPOSITION RobertDuva, Paramus, N.J., assignor to Sel-Rex Corporation, Nutley, N.J., acorporation of New Jersey No Drawing. Filed Aug. 28, 1961, Ser. No.134,106 9 Claims. (Cl. 1061) This invention is concerned with thedeposition of gold. More particularly, the invention relates to thedeposition of gold without the use of applied current.

The chemical reduction and the immersion deposition of many metals uponbasis metals are widely practiced. In the latter process, it is possibleto obtain deposit thicknesses of the order of A2 to A of amicron andonly on basis metals less noble in the electrolyte than the metal beingplated out.

On the other hand, the chemical reduction (electroless) process is notso limited in application as the immersion method, in that the metalwill be continually deposited by chemical reduction to substantialthicknesses, although deposition generally is limited to certaincatalytic basis metals which effectively accept the deposit. Inaddition, suitably activated non-conductors can be plated by electrolesschemical reduction.

The most common method of electroless chemical reduction employs asolution of the ions of the metal to be deposited and a reducing agent.Most commercial processes employ the hypophosphite ion as the reductant;the bath is operated at about 90 C. and certain buffers are added toprevent wide pH fluctuation. However, gold is a highly electronegativeelement, i.e., it is easily reduced, and the system containinghypophosphite is unstable with the result that spontaneous decompositionoccurs and the bath has to be refined.

This invention is based on the discovery of an entirely different systemthat will produce gold deposits at the rate of 2 to 2 /2 microns perhour at a temperature substantially below that ordinarily used foreither immersion or electroless gold. Furthermore, the bath or baths ofthe invention are stable for long periods of time under productionconditions. Thus, it has been found that gold deposits from a solutionthereof in the presence of palladous ions. The palladous ion may bepresent in the bath containing the gold salt or may be applied to thework to be plated from a separate bath.

The bath of the invention is made up of potassium gold cyanide to whichis added 0.01- g./l. of a soluble palladous salt calculated as Pd.Suitable soluble palladous salts include palladium P salt, (NH Pd(NOdisodium palladium tetrachloride, Na PdCl palladous chloride andsulfate.

It is to be understood that the invention is not limited in scope byexplaining a theory upon which the operation is believed to depend. Itis believed that when the simpler soluble Pd salts such as palladouschloride or sulfate are employed in the bath of the invention, a complexpalladous salt (providing Pd++ ions) is formed in said bath which may besimilar in structure to the P salt, for example. In the event that agreat excess of free CN ions exists in said bath no deposition takesplace. It is believed that this effect is due to the formation of a verystable Pd complex which does not provide Pd ions. The ratio of the Pdsalt to free CN- ions should therefore be maintained high enough toprovide Pd++ ions which are necessary for the deposition of the gold. Anexact measurement of the working ratio of Pd salt/CN is not possible butit is very simple to test a given solution to see if gold depositstherefrom. It is understood that the palladium of the P salt and ofdisodium palladium tetrachloride provides Pd++ ions in the gold cyanidebath.

In practicing this invention a bath of the following composition wasprovided:

As indicated in the broad range of the table above, the alkali salts ofweak acids and the weak acid, per se, are optional constituents which,however, improve the stability of the bath. Weak acids are specifiedbecause of their buffering and/ or complexing properties. Satisfactoryweak acids include the phosphoric acids, and substantially all of theorganic acids such as acetic, propionic, kojic, maleic, tartaric,citric, etc. Hydroxy polybasic acids such as citric and polybasic aminoacids such as ethylenediamintetraacetic acid (EDTA) are especiallydesirable because they have a high degree of complexing activity.

The method of plating out the gold is simple in that the bath is heatedto about -75 C. and the clean metal workpiece is inserted for sufficienttime to obtain the desired thickness of gold.

With the bath comprising 4 g./l. of potassium gold cyanide, 25 g./l. oftripotassium citrate, 12 g./l. of citric acid, 2 g./l. of palladium Psalt and sufiicient ammonia to produce a pH of 10, there was obtained agold thickness of micron in One hour, 2 /2 microns in 2 hours, and 3 /3microns in three hours. In other experiments satisfactory deposits wereobtained at the high and low ranges listed in Table I.

To show that the addition of salts of organic acids are not necessary, abath was made up as follows:

KAu(CN) 4.0 Palladium P salt 1.5

A copper panel immersed in this bath at 65 C. received a deposit of 18mg. in 15 minutes, the deposition being matte to semi-bright.

Although satisfactory deposits can be obtained from the basic bathcontaining only a soluble gold cyanide and palladium P salt, (NI-I Pd(NOthe addition of the salts of organic acids such as citric, glycollic,tartaric which are capable of complexing any base metals present in thesystem and thus maintain the bath in good operating condition for alonger period of time, is very desirable. The sodium salts of citric,glycollic, acetic and tartaric acids have been found to producesatisfactory deposits and to improve the operation of the bath similarto the way the citric acid salt additions do.

As further examples, a bath was made up containing:

G./l. KAu(CN) 4 K HPO 20 Palladium P salt l NH4OH to adjust to pH: 10.0.

G./l. KAU(CN) 4 Na PdCl 1 NH OH to pH 10.

A brass panel immersed in this bath received a gold deposit at the rateof 14 mg./in. in 15 minutes.

The process works as well for a properly sensitized non-conductor. Thus,a printed plastic board with copper circuit elements including holestherein was cleaned, etched in cuprous chloride, treated in 50% HCl,sensitized in stannous chloride solution and then catalyzed by immersingin PdCl solution. The board was then immersed in a solution of 4 g./l.of KAu(CN) 1 g./l. of Na PdCh and 40 g./l. of the disodium salt of EDTAneutralized with NH to pH 10. A satisfactory gold deposit was obtainedon the plastic board and through the holes.

The features and principles underlying the invention described above inconnection with specific exemplifications will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims shall not be limited to any specific feature ordetails thereof.

I claim:

1. As a composition for chemically depositing gold an aqueous solutionconsisting essentially of 0.5-30 g./l. of a soluble gold cyanide, 0.01to 30 g./l. of a soluble palladous salt, absent sufficient CN ions toprevent deposition induced by said palladous salt, and sufficient alkalito provide a pH of 8-11.

2. As a composition for chemically depositing gold, an aqueous solutionconsisting essentially of Alkali gold cyanide (calc. as Au) 0.5-30Alkali salt of a weak acid selected from the class consisting of thephosphoric acids and organic acids Up to 300 Weak acid selected from theclass consisting of the phosphoric acids and organic acids Up to 100Palladous salt (calc. as Pd) 0.01-30 Ammonia to pH 8-11.

the CN/Pd++ ratio being below that at which the chemical depositioninduced by Pd++ is prevented from taking place by CN ions.

3. As a composition for chemically depositing gold, an aqueous solutioncontaining Potassium gold cyanide About 4. Trialkali citrate About 25.Citric acid About 12. Palladous salt About 2.

Ammonia to approx. pH 10.

the CN/Pd++ ratio being below that at which the chemical depositioninduced by Pd++ is prevented from taking place by CN ions.

4. The composition as claimed in claim 2 in which the palladous salt is(NH Pd(NO 5. The composition as claimed in claim 2 in which thepalladous salt is disodium palladous tetrachloride.

6. A process for depositing gold on a workpiece having a conductingsurface comprising immersing the workpiece in an aqueous solutionconsisting essentially of 05-30 g./l. of a soluble gold cyanide,sufficient alkali to provide a pH of 8-11, in the presence of 0.01 to 30g./l. of a soluble palladous salt, and in the absence of sufficient CNions to prevent deposition induced by said palladous salt whereupon goldis deposited on said conducting surface by the chemical reductionprocess.

7. A process according to claim 6 wherein the palladous ions areobtained from (NH Pd(NO 8. A process according to claim 6 wherein thepalladous ions are obtained from disodium palladium tetrachloride.

9. A process according to claim 6 wherein the palladous ions areobtained from palladous chloride.

JAMES A. SEIDLECK, Primary Examiner.

J. EVANS, Assistant Examiner.

1. AS A COMPOSITION FOR CHEMICALLY DEPOSITING GOLD AN AQUEOUS SOLUTIONCONISTING ESSENTIALLY OF 0.5-30 G./1. OF A SOLUBLE GOLD CYANIDE, 0.01 TO30 G./1. OF A SOLUBLE PALLADOUS SALT, ABSENT SUFFICIENT CN IONS TOPREVENT DEPOSITION INDUCED BY SAID PALLADOUS SALT, AND SUFFICIENT ALKALITO PROVIDE A PH OF 8-11.
 6. A PROCESS FOR DEPOSITING GOLD ON A WORKPIECEHAVING A CONDUCTING SURFACE COMPRISING IMMERSING THE WORKPIECE IN ANAQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 0.5-30 G./1. OF A SOLUBLEGOLD CYANIDE, SUFFICIENT ALKALI TO PROVIDE A PH OF 8-11, IN THE PRESENCEOF 0.01 TO 30 G./1. OF A SOLUBLE PALLADOUS SALT, AND IN THE ABSENCE OFSUFFICIENT CN IONS TO PREVENT DEPOSITION INDUCED BY SAID PALLADOUS SALTWHEREUPON GOLD IS DEPOSITED ON SAID CONDUCTING SURFACE BY THE CHEMICALREDUCTION PROCESS.